Getting up and Running Quickly

Emerson’s Gergo Kertesz shared a story of how to get a plant started up and running with the aid of control loop tuning. His abstract:

This Russian Petrochemical Company is one of the world’s largest petrochemical complexes. The company specialises in the production and sale of synthetic rubbers. When the polymerization unit was migrated from a pneumatic control system to DeltaV initial regulatory control tuning was tackled as a focused activity within the startup which contributed to achieving full rate quickly and safely. Using this plant as an example, the workshop will focus on how to tackle startup tuning effectively on a new unit and how to make rapid progress if the available time is short.

Gergo opened noting typical problems in the control system controller found during startup with the tuning, control algorithms, and configuration. With the final control element, typically a control valve, hysteresis, valve sizing, and flow characteristics are common spots for issues. With the measurement device, configuration of range and calibration are areas to look at.

Oscillation of control loops may not be seen from the process variable. The valve output may be oscillating greatly while the PV remains fairly steady. Gergo shared how the gain can be scaled back and the reset increased greatly in the PID loop to reduce the oscillations in the control valve.

For measurement noise, Gergo recommended that if possible, do not apply filter time. Significantly reduce the gain (0.5 to 0.01 in his example), increase the reset, and increase the PV filter in the PID loop.

For valve hysteresis, if it is more than 5%, work on the valve since tuning can’t help much. New tuning is risky because of the control delay. The valve needs be maintained or replaced. First measure it. For his example the hysteresis is about 12%. The control will oscillate, because of the additional delay in the control action.

Gergo shared a valve positioning example with a valve oversized for the application in which it was installed. A random movement in the valve creates a big jump in the process variable. Identifying this trend showing these jumps help confirm the need to replace the valve with one more appropriately sized. From the PID control standpoint, a high process gain amplifies these random movements. The gain, reset, and PV filter all needed to be reduced.

Interacting control loops are a challenge. Gergo shared a boiler example where heat coming into the boilers and liquid level control for the boilers. The feed and level controllers interacted with one another. Both loops were oscillating excessively. Gergo put the loops into manual, performed step tests, and modified the two PID loop tuning parameters. For both, gains were slightly reduced, resets significantly reduced, and PV filters added.

Gergo closed noting that all control loops were checked with problems identified and prioritized. Overall variability was reduced 40% and process stability increased.

You can connect and interact with other control optimization specialists in the Operate & Maintain track of the Emerson Exchange 365 community.